The oral route of administration is perhaps the most preferable means of delivering an antigen or pharmaceutically active agent to man. This route does however suffer from the major disadvantage that there is generally poor uptake of antigens or pharmaceutically active agents by the gastrointestinal tract and some agents may be destroyed by prolonged exposure to proteolytic enzymes. In this regard, attemps to orally immunize man or animals in the past have met with limited success. Effective vaccination has generally only been achieved by the administration of large quantities of antigen or by combining parenteral priming with oral boosting. Recent work by us utilizing a number of molecules with the ability to bind to the intestinal mucosa has demonstrated effective oral immunization using low doses of these binding proteins or by coupling various antigens or haptens to these carriers. Uptake and delivery to the circulation of these molecules from the intestine seemed to be due to receptor mediated endocytosis.
It has been known for some time that a number of specific uptake mechanisms exist in the gut for uptake of dietary molecules. Thus there are specific uptake mechanisms for monosaccharides, disaccharides, amino acids and vitamins. Most of these uptake mechanisms depend upon the presence of a specific protein or enzyme such as monosaccharidase or disaccharidase situated in the mucosal lamina propria which binds to the molecule and transports it into the cells lining and lamina propria.
Two notable exceptions to these uptake mechanisms are found with iron transport and VB12 uptake. In both these cases a specific binding protein is released into the intestine, which binds to its ligand in the lumen of the gut.
Thus, during iron uptake in the intestine transferrin is released from the stomach, binds to iron and is in turn bound by a receptor on the duodenal mucosa. The receptor-transferrin-iron complex is then taken up by receptor mediated endocytosis.
Similarly, the absorption of physiological amounts of VB12 by the gut requires that it be complexed with a naturally occurring transport protein known as intrinsic factor (IF) (1-5). This protein is released into the lumen of the stomach by parietal cells in the fundus. Once bound to intrinsic factor, the VB12.IF complex interacts with a membrane bound receptor for IF located on the terminal ileum of the small intestine. The receptor-IF-VB12 complex is then internalized by a process of receptor mediated endocytosis (RME). Allen and Majerus (7) demonstrated that it is possible to chemically modify VB12, couple it to a resin and use the VB12-resin to affinity purify IF. This finding suggested to us that it may be possible to couple large macromolecules (such as the resin used by Allen and Majerus) to VB12 and to still preserve it's ability to interact specifically with intrinsic factor. By coupling molecules to VB12 in such a way as to preserve the ability of VB12 to interact with intrinsic factor it was hoped that we could use the natural uptake mechanism for VB12, to deliver various proteins, drugs or other pharmaceutically active molecules to the circulation.
It is thus the object of this invention to utilize the VB12 uptake mechanism to transport active substances such a drugs, hormones, antigenic material and the like, covalently coupled to VB12 or an analogue thereof, from the intestinal lumen into the circulation.